Photoperiodism is an organism's response to the relative lengths of day and night (i.e., the photoperiod). We have always known that plants are tied to the seasons: each kind of plant forms flowers at about the same time each year; for example, some in spring, some in summer, some in autumn. Botanists knew that plants responded in various ways to temperature and other changes in the environment, but it was not until World War I (1914-18) that anyone tested plant responses to photoperiod. At that time Wightman W. Garner and Henry A. Allard at the U.S. Department of Agriculture in Maryland began to control various parts of the environment in their greenhouses to see if they could make a new hybrid tobacco bloom in summer rather than only in winter. Nothing worked until they put plants into dark cabinets for various times overnight in midsummer. Long...